^Pretty much what the above poster said. Post-interview communication is basically a game. Programs will send different variations of "ranked to match" emails to applicants and applicants will send different variations of "you're my #1" to programs. Both sides have been lied to and burned numerous times before (i.e. an applicant gets a really nice "we'd love to have you here" email from a program, reads too much into it, ranks that place #1 and doesn't match there, or a program ranks an applicant highly after that applicant tells them you're my #1, and that applicant matches elsewhere). Applicants and programs both want to (understandably) match high on their lists so that's why this stuff happens. It's all pretty stupid and I think post-interview communication should just be outright banned all over.
I think #1 emails from applicants might make a small difference at some programs but probably not at the majority of the programs. PDs have no obligation to believe applicants if they've been burned before. They're going to rank applicants the way they want. Although, I will say, there are probably ways to tell when someone is being genuine (i.e. it's a program in their hometown, they want to be really close to family/SO, they've indicated during interviews or letter of interest that the program is #1 for them) but even then, there's no way of being 100% certain.